This application is related to Japanese Patent Application No. 2000-348423 filed on Nov. 15, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave filter, particularly a surface acoustic wave filter in a ladder form in which a plurality of surface acoustic wave resonators are placed in series arms and parallel arms.
2. Description of Related Art
Ladder-form surface acoustic wave (referred to as SAW hereinafter) filters are generally used as SAW filters for portable telephones. In the ladder form SAW filter, a plurality of SAW resonators each composed of an interdigital transducer (referred to as IDT hereinafter) and reflectors disposed on both sides of the IDT are connected in the ladder form on a piezoelectric substrate.
FIG. 18 shows the construction of a basic ladder-form SAW filter currently used. This SAW filter is so constructed that a plurality of SAW resonators are connected in series (S1, S2) and in parallel (P1, P2). FIG. 19 shows a graph of general frequency characteristics required for SAW filters. In FIG. 19, BW1 and BW2 represent band widths at attenuation amounts defined by specifications. For example, BW1 represents a band width at an attenuation amount of xe2x88x923 dB and BW2 represents a band width at an attenuation amount of xe2x88x9220 dB.
Here, the ratio of BW2 to BW1, i.e., BW2/BW1, is called a shape factor and is a criterion indicating the performance of the filter. A smaller shape factor, that is, a shape factor closer to 1, is better. With recent development in mobile communication systems, filters with a better shape factor are demanded.
However, the shape factor obtained with the basic ladder-form SAW filter shown in FIG. 18 is substantially determined by an electromechanical coupling coefficient of a material for a substrate used. Therefore, it is difficult to improve the shape factor simply by arranging SAW resonators in series and in parallel as shown in FIG. 18.
Accordingly, there is a problem in providing filters having such an excellent shape factor as demanded now with regard to ladder-form SAW filters using SAW resonators as shown in FIG. 18.
The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to improve the shape factor of SAW filters and provide SAW filters having an improved shape factor.
The present invention provides a SAW filter comprising SAW resonators each composed of an IDT and reflectors disposed on both sides of the IDT, the SAW resonators being arranged in series arms and in parallel arms to be connected in a ladder form on a piezoelectric substrate, wherein at least one of the SAW resonators is replaced with an IDT that does not involve resonance in a propagation direction of a surface acoustic wave.
With this construction, the shape factor of the SAW filter can be improved, and also the SAW filter can be reduced in size.
Here, the IDT that does not involve the resonance may be formed of split electrodes.
Also, the IDT that does not involve the resonance may be connected only to a series arm of a ladder-form structure or only to a parallel arm of the ladder-form structure. Further, a plurality of IDTs that do not involve the resonance may be disposed in one or more of the series arms and in one or more of the parallel arms.
The present invention also provides a SAW filter comprising a piezoelectric substrate; and a plurality of SAW resonators and at least one IDT that does not involve resonance, the SAW resonators and the IDT being formed on the piezoelectric substrate and connected in a ladder form, wherein the SAW resonators are each composed of an IDT and reflectors disposed on both sides of the IDT in a direction parallel to a propagation direction of a surface acoustic wave, and the IDT that does not involve the resonance is formed of split electrodes.
In the viewpoint of size reduction, the split electrode may preferably be formed of comb-form electrodes including pairs of two adjacent electrode fingers which are alternately arranged in a direction parallel to the propagation direction of the surface acoustic wave
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.